IDO is a well-described mechanism of tumour immune evasion in mice [28] and shows direct effects in human T cells [30–32], though a phase 3 trial of combination IDO inhibitor and programmed cell death protein 1 (PD-1) inhibition recently failed to improve progression-free survival compared to PD-1 inhibitor immunotherapy alone (clinicaltrials.gov identifier: NCT02752074). The gene discussed is IDO1; the disease is neoplasm.